1. Field of the Invention
The present invention relates generally to pulse power sources and more particularly, to an apparatus and method for eliminating secondary pulses due to stage mismatch in nonlinear magnetic compression pulse power sources for gas discharge lasers.
2. Description of the Prior Art
In applications where pulsed lasers are operated for extended periods of time, there is a need for energy efficiency, long term reliability and high component lifetimes. In these applications, magnetic compression circuits in conjunction with solid state switches have recently been utilized to supply pulse power because of the reliability and useful life of the magnetic compression circuits and solid state switches.
Such magnetic compression circuits generally utilize a multi-stage LC network to provide power compression. They are generally in accordance with U.S. Pat. No. 5,313,481.
One particular difficulty with using multiple stages of magnetic compression is the inability to perfectly match one stage to the next, so that complete energy transfer from one stage to the next cannot be accomplished. This problem is compounded when one or more stages are located in different thermal environments. The high temperature sensitivity of the capacitors typically used in these high voltage systems causes mismatch between stages as the temperature of each stage reaches a different value.
The mismatch between successive compression stages will cause some of the energy in the forward traveling pulse to be left behind at each compression stage. This energy is temporarily stored in the capacitors of each stage. Under certain conditions, this left-over energy can form a second electrical pulse which is launched toward the load some time after the main pulse. Such secondary pulses can, for example, cause damage to the electrodes in a gas discharge laser.
FIG. 1 illustrates a typical pulse power system which implements a magnetic pulse compression circuit with stages that exhibit some small mismatch. In the system of FIG. 1, C.sub.0, L.sub.0, and C.sub.1 form one C-L-C circuit while C.sub.1, L.sub.1 and C.sub.2 form another C-L-C circuit. In the first C-L-C circuit, C.sub.0 is the source capacitor, and in the second C-L-C circuit, C.sub.1 is the source capacitor. During normal operation, a command charge power supply is used to charge the DC storage capacitor, C.sub.0, to the desired voltage. Upon command, a semiconductor switch such as an Isolated Gate Bipolar Transistor (IGBT), or silicon controlled rectifier (SCR) switch will discharge C.sub.0 (through L.sub.0, which limits the peak current), into the capacitor C.sub.1 at the input to the nonlinear magnetic pulse compression network. The nonlinear magnetic compression network provides temporal compression of the stored energy via the large inductance changes associated with the saturation of the nonlinear magnetic core material which is contained within the induction cores.
Once the compressed pulse reaches the load, such as a gas discharge laser, the mismatch between the load and the output stage leads to some reflected energy traveling backward through the compression stages. This reflected energy is captured on C.sub.0 through the use of a semiconductor switch which will carry current in only one direction. This keeps the energy on C.sub.0 from again traveling forward through the compression stages. In an ideal system, the capacitance values in each stage are accurately matched so that there is no overshoot or undershoot in the voltage of the source capacitor in a C-L-C circuit that is provided by an adjacent stage.
As mentioned above, the values of C.sub.0, C.sub.1, and C.sub.2 do not remain at the same values as the temperature of each stage changes. When capacitive mismatch exists, the voltage on the source capacitor will not ring down to zero voltage upon the transfer of energy to an adjacent stage. If the source capacitor is slightly larger than desired, the voltage on the source capacitor will ring down to a non-zero positive value. If the source capacitor is slightly smaller than desired, the voltage will overshoot the zero voltage point and end up with a negative value.
This overshoot or undershoot voltage will occur for both the main pulse traveling toward the load and for the backward traveling reflected energy pulse. The voltage on the capacitor of a particular stage will create a potential across its neighboring inductor. Since each inductor, except for L.sub.0, is a saturable inductor, the saturable inductors in the compressor circuit will eventually saturate in the direction necessary to allow energy transfer of the secondary pulse toward the load.
FIG. 2 is a graph of the voltage on capacitors C.sub.0 and C.sub.1 as a function of time, when C.sub.0 is slightly undersized relative to C.sub.1. In this case, the voltage on C.sub.0 will overshoot negatively during the initial energy transfer from C.sub.0 to C.sub.1. The reflected energy pulse travels from the load, back through the various stages, to C.sub.1, and finally back to C.sub.0. During the transfer of charge from C.sub.1 back to C.sub.0, the voltage on C.sub.1 will undershoot the zero voltage point because its capacitance value is slightly larger than that of C.sub.0. This results in leaving a negative voltage on C.sub.1 since the reflected energy pulse is a negative voltage pulse.
This leftover negative voltage will remain on C.sub.1 until L.sub.1 saturates in the reverse conduction direction, either due to the potential difference between C.sub.1 and C.sub.2, or because of the bias current used to reset each saturable inductor to its proper point on the B-H curve in preparation for the next pulse. Once L.sub.1 reverse saturates, a negative voltage pulse is launched toward the load which will be compressed by the remaining stages. In extreme cases, this pulse can have a magnitude of 1 to 2 kV compared to the main pulse peak of 15-20 kV. A pulse of such a magnitude can cause significant erosion to the electrodes in a gas discharge laser and result in lower electrode lifetimes.
Great care is taken to prepare the laser for the main 15-20 kV pulse. The gas medium has been cooled and homogenized, or in some cases replaced since the previous main discharge event. Just prior to the main discharge, care is taken to uniformly pre-ionize the medium. The end result is a discharge which initially will uniformly fill the discharge chamber. The plasma draws current from a large electrode area. Through processes such as carrier attachment, the discharge begins to localize into streamers over time periods measured in tens of nanoseconds. These processes are capable of reducing the discharge to a single localized arc in a few 100 nanoseconds. A few millijoules delivered into such a localized area can be more destructive than a few joules delivered to a uniform plasma which fills the discharge volume. Consequently, it is very important to minimize any energy pulses which might be delivered to the discharge other than at the desired main discharge time.
Accordingly, there is a need in the technology to provide an apparatus and method for eliminating reflected energy due to stage mismatch in pulse power lasers, so that electrode erosion may be minimized.